Heterostructured Core–Shell Ni–Co@Fe–Co Nanoboxes of Prussian Blue Analogues for Efficient Electrocatalytic Hydrogen Evolution from Alkaline Seawater

The rational construction of efficient and low-cost electrocatalysts for the hydrogen evolution reaction (HER) is critical to seawater electrolysis. Herein, trimetallic heterostructured core–shell nanoboxes based on Prussian blue analogues (Ni–Co@Fe–Co PBA) were synthesized using an iterative coprecipitation strategy. The same coprecipitation procedure was used for the preparation of the PBA core and shell, with the synthesis of the shell involving chemical etching during the introduction of ferrous ions. Due to its unique structure and composition, the optimized trimetallic Ni–Co@Fe–Co PBA possesses more active interfacial sites and a high specific surface area. As a result, the developed Ni–Co@Fe–Co PBA electrocatalyst exhibits remarkable electrocatalytic HER performance with small overpotentials of 43 and 183 mV to drive a current density of 10 mA cm–2 in alkaline freshwater and simulated seawater, respectively. Operando Raman spectroscopy demonstrates the evolution of Co2+ from Co3+ in the catalyst during HER. Density functional theory simulations reveal that the H*–N adsorption sites lower the barrier energy of the rate-limiting step, and the introduced Fe species improve the electron mobility of Ni–Co@Fe–Co PBA. The charge transfer at the core–shell interface leads to the generation of H* intermediates, thereby enhancing the HER activity. By pairing this HER catalyst (Ni–Co@Fe–Co PBA) with another core–shell PBA OER catalyst (NiCo@A-NiCo-PBA-AA) reported by our group, the fabricated two-electrode electrolyzer was found to achieve high output current densities of 44 and 30 mA cm–2 at a low voltage of 1.6 V in alkaline freshwater and simulated seawater, respectively, exhibiting remarkable durability over a 100 h test.

as the substrate for the working electrode. A Pt mesh and saturated Ag/AgCl/Clwere employed as the counter electrode and reference electrode, respectively. An aqueous solution of 1 M NaOH was the electrolyte. After the activation, the working electrode was taken out from the electrolyte and dried at room temperature. NiCo@A-NiCo-PBA-AA was scratched off the FTO substrate as the anode material for water splitting. [1] Characterizations Powder X-ray diffraction (XRD) patterns were collected on a Bruker D2 ADVANCE diffractometer with Cu K α radiation (λ=1.5418Å). The structure and morphology of the samples were characterized by field-emission scanning electron microscopy (FESEM, Zeiss LEO 1525) and transmission electron microscopy (TEM, JEOL-2100Plus, JEOL-2100F). Energy-dispersive X-ray spectroscopy (EDS) attached to the TEM was used to analyze the composition of the nanoscale samples. Fourier-transform infrared (FT-IR) spectra were collected on a Thermo Scientific Nicolet iS50 FT-IR spectrometer fitted with a diamond ATR module, 4000-400 cm -1 , 64 scans, and 0.5 cm -1 resolution. The pore structures of samples were characterized using the N 2 adsorption/desorption isotherm tested on the Micromeritics 3 Flex Physisorption at 77 K. The specific surface area was determined by the multi-point Brunauer-Emmett-Teller (BET) method and the pore-size distribution was calculated based on the Barrett-Joyner-Halenda (BJH) method. X-ray photoelectron spectroscopy (XPS) analysis was conducted on a PHI-5000 VersaProbe X-ray photoelectron spectrometer using an Al K α X-ray source. The powder samples were stuck onto the specific sample holders using conductive double-sided carbon tapes for the test. Continuous wave electron paramagnetic resonance (EPR) spectra were recorded at X-band (ca. 9 GHz) on a Bruker EMX Micro spectrometer equipped with a Bruker ER4112SHQ resonator at room temperature. Samples were placed in quartz EPR tubes (4 mm OD, 3 mm ID) in identical quantities, placed at the same optimal position in the cavity and measured under non-saturating conditions. G values were obtained by comparison with a Bruker Strong Pitch standard (g = 2.0028). Operando Raman spectra were obtained with an inVia Renishaw confocal Raman microscope operated with an incident laser beam at 532 nm focused through a 50x objective (Leica). The laser intensity was set to < 1 mW and Raman spectra were collected in static mode, with an exposure time of a few seconds every 5 minutes to minimize sample heating. To monitor the evolution of catalyst samples during HER process in alkaline freshwater, each Raman spectrum was collected after a constant potential was applied to the catalyst electrode for 5 min. Each Raman spectrum was obtained using an integration time of a few seconds, accumulating 5 times. The laser shutter remained closed between spectrum collections.

Electrochemical measurements
The electrochemical tests of the materials were performed using a Metrohm Autolab electrochemical workstation PGStat-12 (Utrecht, the Netherlands) connected to a three-electrode cell. A glassy carbon electrode (GCE) of 3 mm diameter served as the substrate for the working electrode, and a graphite rod and a Hg/HgO/OHelectrode were employed as the counter electrode and reference electrode, respectively. 5 mg of the as-prepared PBA catalyst was dispersed in 4.5 mL of a water/isopropanol solution (1:3) containing 500 μL Nafion (5%). The resulting solution was sonicated for 0.5-1 h. When the solution was well dispersed, 4 μL of the above solution was frequency range from 10 kHz to 0.01 Hz. CV curves with different scan rates (10-60 mV s -1 ) were measured over a potential range in which redox processes were absent to calculate the electrochemical double-layer capacitance: is the double-layer capacitance (F cm -2 ) of the electroactive materials, j a and j c is the anodic and cathodic current density (mA cm -2 ), respectively, recorded at the middle of the selected potential range, and v is the scan rate (mV s -1 ). All results reported in this work were converted to the RHE scale according to the Nernst equation, The HER activity was obtained after the iR-correction to the LSV. Typically, the iR-correction is according to the following equation, where E is the potential after iR-correction, E RHE is the measured potential referred to RHE, i represents the measured current, and R is the uncompensated resistance which could be determined by electrochemical impedance spectroscopy (EIS). The uncompensated resistance is found as the real impedance where the imaginary part of the impedance is zero in a Nyquist plot.
A water-splitting device with a two-electrode configuration was assembled. The cathode and the anode electrodes were made by depositing Ni-Co@Fe-Co PBA and NiCo@A-NiCo-PBA-AA onto Ni foam (2 × 1 cm 2 ) and then drying in air. To obtain a total catalyst loading of approximately 1 mg cm -2 , the deposition process was repeated several times. Then, the Ni foams loaded with catalysts were used as both cathode and anode for water electrolysis.

Computational details
Density functional theory (DFT) calculation was performed using the generalized gradient approximation (GGA) Perdew-Burke-Ernzerhof (PBE) functional, and the projected augmented plane-wave method implemented in the Vienna ab initio simulation program (VASP) software code.           The η 10 overpotentials and Tafel slopes of Pt/C in alkaline freshwater and simulated seawater were 68 mV and 58 mV dec -1 , and 194 mV and 69 mV dec -1 , respectively. This showed that the activity of Pt/C was lower than that of Ni-Co@Fe-Co PBA, and was comparable to that of NiFe-Co PBA in the two alkaline electrolytes.
15             The η 10 overpotentials and Tafel slopes of nickel foam for HER and OER in alkaline freshwater were 476 mV and 142 mV dec -1 , and 507 mV and 110 mV dec -1 , respectively, which were much higher than the those of the PBA catalysts, and thus the contribution of nickel foam to water splitting activity was very limited. The η 10 overpotential of Ni-Co@Fe-Co PBA for OER was 428 mV in alkaline freshwater ( Figure   S15a). The Ni-Co@Fe-Co PBA//Ni-Co@Fe-Co PBA electrode couple reached a current density of 10 mA cm -2 at a cell voltage of 1.6 V in alkaline freshwater ( Figure S15b), which was significantly inferior to that of the Ni-Co@Fe-Co PBA//NiCo@NiCo-PBA-AA electrode couple.